1. Field of the Invention
The present invention generally relates to magnetic heads for tape and disk drives and, more particularly, to magnetic heads that include magnetoresistance (MR) sensors for reading data from storage media.
2. Relevant Background
Computer systems generally utilize auxiliary memory storage devices (e.g., tape drives, disk drives) having media (e.g., magnetic tape, magnetic disks) on which data can be written and from which data can be read for later use. For instance, some storage devices include one or more magnetic heads including read and/or write sensors for performing reading and writing operations on storage media. In the case of a disk drive, one or more magnetic heads are used to read data from and/or write data to concentric, radially spaced tracks on the disk surfaces. In the case of a tape drive, one or more magnetic heads are used to read data from and/or write data to parallel, laterally spaced tracks on the tape surface.
To manufacture such magnetic heads, a multiplicity of magnetic heads are typically simultaneously fabricated by coating a series of layers upon the surface of a wafer that is formed from a head substrate material. The wafer is then separated into individual magnetic heads. The surface formed by one of the separations is polished to form an air-bearing-surface (ABS) or tape-bearing-surface (TBS) of the magnetic head. Each magnetic head includes a write and/or read portion, where the write portion includes at least one write element or sensor and the read portion includes at least one read element or sensor.
MR read sensors (e.g., incorporated on MR heads) are now widespread due to the capability of MR heads to read data at a greater linear density than that of the previously used thin film inductive heads. Generally, an MR sensor detects a magnetic field on the media through a change in resistance in its MR sensing layer (e.g., an “MR element”) as a function of the strength and direction of the magnetic flux being sensed by the MR layer. Some MR sensors operate on the basis of the anisotropic MR (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization of the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant MR (GMR) sensor that manifests the GMR effect. In GMR sensors, the resistance of the MR sensing layer(s) varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers. In the case of a read portion of a magnetic head, the GMR sensor is sandwiched between first and second nonmagnetic electrically insulating read-gap layers, which are in turn sandwiched between first and second ferromagnetic shield layers. Magnetic flux from the surface of the media (e.g., magnetic disc or tape) causes rotation of the magnetization vector of the MR sensing layer(s), which in turn causes a change in electrical resistivity of the GMR sensor. The change in resistivity of the GMR sensor can be detected by passing a current through the GMR sensor and measuring a voltage across the GMR sensor. External circuitry then converts the voltage information into an appropriate format and manipulates that information as necessary.
A further type of MR sensor is the tunneling MR (TMR) read sensor that includes a nonmagnetic insulating barrier layer sandwiched between a ferromagnetic reference layer and a ferromagnetic sense layer. The thickness of the barrier layer is chosen to be less than the mean free path of conduction electrons passing through the TMR read sensor. The magnetization of the reference layer is pinned in a direction perpendicular to the ABS, while the magnetization of the sense layer is oriented in a direction parallel to the ABS. When passing the sense current through the TMR read sensor, the conduction electrons are scattered at lower and upper interfaces of the barrier layer. When receiving a magnetic field emitting from data in the selected data track, the magnetization of the reference layer remains pinned while that of the sense layer rotates. Scattering decreases as the magnetization of the sense layer rotates towards that of the reference layer, but increases as the magnetization of the sense layer rotates away from that of the reference layer. This scattering variation induces a tunneling effect characterized by a change in the resistance of the TMR read sensor in proportion to the magnitude of the magnetic field and cos θ, where θ is an angle between the magnetizations of the reference and sense layers. The change in the resistance of the TMR read sensor is then detected by the sense current and converted into a voltage change that is processed as a read signal.
During a read operation, the first and second shield layers attempt to ensure that read sensor only reads the information stored directly beneath it on a specific track of the magnetic media by absorbing stray magnetic fields emanating from adjacent tracks and transitions. Specifically, a typical shield includes a plurality of magnetic domains (e.g., areas, regions, etc.) separated from each other by a plurality of magnetic domain walls. Each magnetic domain has a magnetization that is oriented in a direction different than that of adjacent domains. The application of an external magnetic field (e.g., from an adjacent track or transition of the magnetic storage medium during operation, during manufacture, etc.) to a shield layer and/or stress due to heating can cause the magnetization of each of the domains within the layer to rotate, thereby causing the domains to move (e.g., shift, flop back and forth, etc.). Because of the random nature of the domain wall locations, the domain walls generally do not return to their original location after the external magnetic field is removed.